Lightbox Project

Hey all,
I'm building an ISO A1 sized lightbox for university. The illuminated surface measures 650mm x 900mm. It is made of wood, with a thick perspex top (the surface for tracing), and stainless steel grates on all 4 sides for heat ventilation. Traditionally, these are illuminated by using 2 or more fluorescent tubes. The trouble with this is that they have to be plugged into a socket, get very hot, and wreck your eyes after about 40 mins. Not much help if you're going to be tracing architectural plans for about 6 hours straight! they also usually end up quite bulky, as the perspex has to be about 200mm away from the lamps or else it melts.
Basically, i was thinking of making one using LEDs. They're easy enough to diffuse, and if i used white or blue ones, they'd probably be bright enough to trace through (its one blank a1 sheet on top of another). I suggested the idea to a friend, and he estimated 100 LEDs, but he doesn't actually realise how bright they can be! I estimate a number closer to 15-20. (21 would give 3 rows of 7, which i think if diffused properly, should be ok)
So my big problem at the moment is trying to figure out a power source. Ideally, i'd like to use a 9v battery, or several 9v batteries, and totally avoid using a plug. The studio tends to lack in free sockets when it comes to projects and everyone's got their laptops plugged in!
Can anyone help me come up with a good power source and give me an idea of what kind of resistors i'd need. The perfect setup i'd like to have is an on/off switch and a dimmer.
Thanks all,
tm

 

The LED idea may be quite good, but really isn't practical using batteries. Your white LED's have narrow beams, so you may easily find you'll need more than 21. At 10 - 20 milliamps/LED, that is a lot of current for a battery to handle. Say you have them in groups of three (in series) - the total current will be 7 x .015 (using 15 mills as an average), or 105 milliamps. That is way too much for 9 volt batteries to handle.

You would need a 12 volt gel cell to run the LED's, plus the battery charger. The most common size is a 7 amp hour size for about $20. Don't have any charger figures close to hand about a charger.

The interesting possibility is that you could use an LM317 regulator for an adjustable voltage regulator and be able to vary the intensity of illumination.

 

To determine how much energy you need, you need to specify the amount of light you need. This is not trivial. You need a certain total output in lumens.

A typical computer LCD display has a brightness of around 300 nits = 87.6 foot-lamberts. A foot-lambert is equivalent to a lumen/m^2. You have 0.585 m^2 surface area, so you will need about 51.246 lumens. A high-brightness LED puts out about 60 lumens when drawing about 1.2W.

6 Hours at 1.2W. = 7.2W-hours. If this energy is drawn from a 9V battery, you would need 133mA. 6 hours at this current is 800mA-h. A single 9V battery has about 550mAh capacity, so you would need at least 2.
-----------------------
Some huge caveats to this set of calculations.
1. You probably need more than 300nits to see through multiple sheets.
2. This is assuming 100% light efficiency. This means that all light produced goes to the screen. This is not really possible.
3. This is assuming 100% energy efficiency. You will likely get less than 80%.

This may or may not invalidate the battery usage idea, but should get you on the right track.

 

well, the led's i was thinking about were high efficiency white (or blue) ones with a projection angle of 70 degrees. Thats pretty wide by LED standard. Those diffused might spread the light well.

Just now, i did think to myself, i do have a few 12v plugs at home from old cd walkman chargers. Im not sure if they're AC or DC. If they ARE DC, i could probably just cut off the end (the part that fits into the charger port on the old walkman's) and have a positive and a negative, couldn't i? The cable from the adaptor itself is just 2 wires stuck together, much like a set of headphones, where the 2 insulated wires are stuck together (one for left, one for right). i cant check right now, cos its at home and im in student dorms at the moment. (sorry if i sound like a noob, i'm fairly new to electronics. All the soldering/electronics experience i have is from moding/repairing guitars, and i just solder what the instructions tell me )

The plug looks something like this:

Im thinking that would possibly be easier, cos it eliminates the battery life problem, right?

 

Yes, that would easily do the trick. The 1.5 amp output rating gives plenty of capacity.

With a 12 volt supply, and about 2.1 volts per LED (you could look into our Ebook and read up on LED's if you wish), it would be nice to organize them in strings of 5. The total voltage per LED adds to 10.5, leaving 1.5 volts to supply current. Also assuming 20 ma/LED (that would be a maximum rating), a 75 ohm resistor in each string would limit current nicely. Use 1/2 watt size.

The illumination you get to squint against depends on the diffuser between the source and you. Part of that is the sheets of paper, but a layer of translucent plastic under the paper sounds good too - helps to eliminate bright spots. Depending on thickness, it will decrease the intensity by some amount. that makes it hard to guess the "right" number of LED's. The quality of illumination will be different - white LED's give a soft light. The ones you are likely to get are blue with a yellow scintillation layer, so the "white" has a blue cast to it.

If you wish to control the intensity, you could either switch off rows of LED's, or use an adjustable regulator. The number of LED's per string would need to be reduced.

So, several decisions to make, but an interesting project. I used to use a lightbox before graduating to CAD, so I know what you are going through.

 

well, seeing as how i think using a plug is the best, i'd probably go for 4 rows of 7 (or 7 rows of 4 rather).
I found a site that showed how abrasing the surfaces of the LEDs with sandpaper, till they appear frosted, diffuses the light from the leds. I could get frosted perspex, which is more expensive, but i guess would do the job.
So the circuit will be basically:
+ > Pot > series of sets of LEDs with resistor in each set > -
right?
Or, is it easier to work out if i know the exact LEDs im going for? (i'll be ordering probably from www.radionics.ie , but i have yet to have a look there)

EDIT:
seeing as how you know what fun lightboxing is......... cough cough..... what do you reckon the effectiveness of tracing with BLUE leds would be? I know blue is the next best thing to natural sunlight for improving moods. (Some places actually just have huge blue led pannels turned on in offices during winter!!) Might make lightboxing less tedious....

 

You could throw in a few rows of blue LED's to add some blue. I find that blue LED's have a "piercing" quality to the light that is uncomfortable at a high level.

I would recommend using an adjustable voltage regulator instead of a pot to dim the LED's. It may seem more trouble, but the result will be very satisfactory. Get the data sheet for an LM317, and you will see that two capacitors, a 240 ohm fixed resistor, and a 5000 ohm pot all all it needs to function. A heat sink on the tab will be a good idea - look for some slide-on types in the catalog.

The maximum output from the LM317 will be less than 12 volts. Being optimistic, I'll say about 10.5 volts. That will be the voltage across each string. 4 LED's will drop 8.4 volts, leaving 2.1 volts. limiting current to 20 milliamps will require 105 ohms. It's best to play safe, so use 110 ohms (standard value). The power dissipated by each resistor will be 42 milliwatts. I'd still use 1/2 watt resistors, but 1/4 watt will be enough. I would use 1/2 watt size for the 240 ohm resistor in the regulator.

By all means, shop for price when getting the LED's. White and blue are still much more pricey than the other colors.

 

ok. that gives me a good place to start off anyway. I'll have a look this weekend at what i have at home, and see from there. The white LED's i'll be going for are these ones:
http://radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=3106707

And the blue ones:
http://radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2359938

I'll probably have 1 blue per 5 LED's (4 white, 1 blue). I really am a beginner when it comes to circuitry, so i'll probably keep it as simple as possible. Will i have to have them in series or parallel, and what about resistors?

 

Use one 110 ohm resistor and four LED's in a series string. The strings go in parallel. I'll make up a diagram in a few hours - have to get busy in the real world for a bit.

 

if you want to hang on till the weekend, i'll find out EXACTLY what power supplies i have. i'm sure that would make it easier.